This invention relates to an injection device of an injection molding machine, and more particularly, to a high pressure injection molding device for carrying out high precision molding operations.
In a usual injection molding operation, the injection operation is carried out with the maximum pressure of about 1400-1800 Kg/cm.sup.2 during a material injection process and a pressure maintaining process after the injection process. However, recently, a high pressure injection molding has been performed with an increased injection pressure of about 2500-3500 Kg/cm.sup.2 thereby to obtain mold products of excellent qualities each provided with less weld lines or marks and less irregularly recessed portions on the surface of the mold product due to shrinkage of the injected material in a mold.
With the conventional high pressure injection molding operation, the high injection pressure has been obtained by a method of (a) reducing a diameter D of a screw means of the injection device, (b) increasing a diameter of a piston of an injection hydraulic cylinder assembly, or (c) increasing a hydraulic pressure of a hydraulic drive source. In the injection molding operation, the maximum injection pressure P.sub.max is obtained by the following equation. ##EQU1## where P.sub.0 represents an injection pressure of a hydraulic piston-cylinder assembly of the injection device and D designates a diameter of the screw of the injection device, thus the maximum pressure P.sub.max being in inverse proportion to the square of the screw diameter D.
Moreover, the maximum injection amount W.sub.max of a material and the maximum injection ratio Q.sub.max are represented as follows. ##EQU2## where S is an injection stroke of a piston rod of the hydraulic piston-cylinder assembly, .gamma. is a density of the material, and .eta. is an injection efficiency. ##EQU3## where V is an injection speed of the piston rod.
As is found from these equations (2) and (3), the maximum injection amount W.sub.max and the maximum injection rate Q.sub.max are both in proportion to the square of the screw diameter D.
Accordingly, from these equations (1) through (3), it will be found that with the method (a), described hereinbefore, the maximum injection pressure P.sub.max is increased by reducing the screw diameter D, i.e. heating cylinder, which adversely results in decreasing of the injection amount and the injection ratio. Thus, the plasticizing capability and the injection ratio are lowered thereby to reduce the product manufacturing efficiency, thus being uneconomical. Regarding the method (b), there are the same defects or problems as those mentioned with respect to the method (a) because the maximum injection pressure P.sub.max is increased in proportion to the square of the diameter of the piston of the hydraulic piston-cylinder assembly of the injection device, but the maximum injection ratio Q.sub.max is decreased in inverse proportion to the square of the diameter of the piston. Regarding the method (c), the maximum pumping pressure of a usual high precision vane pump is about 140-150 Kg/cm.sup.2 or that of a usual piston pump is about 190-210 Kg/cm.sup.2, so that a hydraulic apparatus having an increased pumping capacity or piping elements having enlarged sizes are required in order to obtain the extremely increased maximum pumping pressure of the hydraulic source. These facts result in an increase of the constructional cost of the injection molding machine, and in addition, the maintenance of the hydraulic circuit is made extremely difficult and the electric power consumption is inevitably increased for operating the hydraulic source.
Accordingly, as described hereinbefore, the conventional methods for obtaining the high injection pressure for the injection device have various defects or problems for obtaining the desired high injection pressure with the high efficiency and the economical advantage.